The authority of a head of government, such as a president, chancellor, or prime minister and the relationship between that position and other state institutions, such as the relation between the head of state and of the legislature, varies greatly among sovereign states, depending largely on the particular system of the government that has been chosen, won, or evolved over time.

In presidential republics or in absolute monarchies, the head of state is also usually the head of government. The relationship between that leader and the government, however, can vary greatly, ranging from separation of powers to autocracy, according to the constitution (or other basic laws) of the particular state.

In semi-presidential systems, the head of government may answer to both the head of state and the legislature, with the specifics provided by each country's constitution. A modern example is the present French government, which originated as the French Fifth Republic in 1958. In France, the president, the head of state, appoints the prime minister, who is the head of government. However, the president must choose someone who can act effectively as an executive, but who also enjoys the support of the France's legislature, the National Assembly, to be able to pass legislation. In some cases, the head of state may represent one political party but the majority in the National Assembly is of a different party. Given that the majority party has greater control over state funding and primary legislation, the president is in effect forced to choose a prime minister from the opposition party to ensure an effective, functioning legislature. In this case, known as cohabitation, the prime minister, along with the cabinet, controls domestic policy, with the president's influence largely restricted to foreign affairs.

In directorial systems, the executive responsibilities of the head of government are spread among a group of people. A prominent example is the Swiss Federal Council, where each member of the council heads a department and also votes on proposals relating to all departments.

A common title for many heads of government is prime minister. This is used as a formal title in many states, but also informally a generic term to describe whichever office is considered the principal minister under an otherwise styled head of state, as minister — Latin for servants or subordinates — is a common title for members of a government (but many other titles are in use, e.g. chancellor and secretary of state). Formally the head of state can also be the head of government as well (ex officio or by ad hoc cumulation, such as a ruling monarch exercising all powers himself) but otherwise has formal precedence over the Head of Government and other ministers, whether he is their actual political superior (ruling monarch, executive president) or rather theoretical or ceremonial in character (constitutional monarch, non-executive president). Various constitutions use different titles, and even the same title can have various multiple meanings, depending on the constitutional order and political system of the state in question.

As political chief

In addition to prime minister, titles used for the democratic model, where there is an elected legislative body checking the Head of government, include the following. Some of these titles relate to governments below the national level (e.g., states or provinces).

The other Hindustani term generically used for 'Prime Minister'(now used officially only in Pakistan with Urdu as official language) : वज़ीर-ए-आज़म/وزیر اعظم (translit. Wazīr-ē-Āzam), lit.:'Grand Vizier/Prime Minister'

For 'Prime Minister of India' : भारतीय प्रधानमन्त्री/भारत के प्रधानमन्त्री (translit. Bhāratiya Pradhānamantrī/Bhārat Kē Pradhānamantrī), translation:'Indian Prime Minister/Prime Minister of India'(this term is used by the Government of the Union and the State Governments of India, under the umbrella of "Hindi Language");

For 'Prime Minister of Pakistan': وزیر اعظم پاکستان/پاکستان کے وزیر اعظم (translit. Wazīr-ē-Āzam Pākistān/Pākistān Kē Wazīr-ē-Āzam), This is the term used in India and Pakistan under the umbrella of Urdu, the Hindi term being, पाकिस्तानी प्रधानमन्त्री/पाकिस्तान के प्रधानमन्त्री (translit.Pākistānī Pradhānamantrī/Pākistān Kē Pradhānamantrī)

Historically, various terms like Pradhānamantrī, Pradhān, Pantapradhān, Sadr-ē-Riyāsat, Sadr, Wazīr-ē-Āzam, Wazīr-ē-Ālā, Mahāmantrī, Wazīr-ē-Khazānā, Pēśwā, Dīwān, Dīwān Sāhib, Dīwān Bahādur, Dīwān Pramukh, Sadr-ul-Maham, Pantapramukh, Ālāmantrī, etc. have been used by various Empires, Kingdoms and Princely States of India as a title for the Prime Minister, some of these titles were also used by the sovereign of various kingdoms.

Malay: In Malaysia, the head of government of the constituent states are expressed in the Malay language (either Ketua Menteri, "chief minister" in the Malaysian states without a monarchy (Malacca, Penang, Sabah and Sarawak), or Menteri Besar " great minister" in the sultanates and other monarchic states).

Under a dominant head of state

In a broader sense, a head of government can be used loosely when referring to various comparable positions under a dominant head of state (especially is the case of ancient or feudal eras, so the term "head of government", in this case, could be considered a contradiction in terms). In this case, the prime minister serves at the pleasure of the monarch and holds no more power than the monarch allows. Some such titles are diwan, mahamantri, pradhan, wasir or vizier.

Indirectly referred as the head of state

In some cases, the head of state is a figurehead whilst the head of the government leads the ruling party. In some cases a head of government may even pass on the title in hereditary fashion. Such titles include the following:

Control over or ability to vote down fiscal measures and the budget (or supply); a government is powerless without control of the state finances. In a bicameral system, it is often the so-called lower house, e.g. the British House of Commons that exercises the major elements of control and oversight; in some others, e.g. Australia and Italy, the government is constitutionally or by convention answerable to both chambers/Houses of Parliament.

All of these requirements directly impact the Head of government's role. Consequently, they often play a 'day to day' role in parliament, answering questions and defending the government on the 'floor of the House', while in semi-presidential systems they may not be required to play as much of a role in the functioning of parliament.

Appointment

In many countries, the Head of government is commissioned by the Head of state to form a government, on the basis of the strength of party support in the lower house, in some other states directly elected by parliament. Many parliamentary systems require ministers to serve in parliament, while others ban ministers from sitting in parliament; they must resign on becoming ministers.

Removal

Heads of government are typically removed from power in a parliamentary system by

Dismissal — some constitutions allow a Head of state (or their designated representative, as is the case in some Commonwealth countries) to dismiss a Head of government, though its use can be controversial, as occurred in 1975 when then Australian Governor-General, Sir John Kerr, dismissed Prime Minister Gough Whitlam in the Australian Constitutional Crisis.

Death — in this case, the deputy Head of government typically acts as the head of government until a new head of government is appointed.

First among equals or dominating the cabinet?

Constitutions differ in the range and scope of powers granted to the head of government. Some older constitutions; for example, Australia's 1900 text, and Belgium's 1830 text; do not mention their prime ministerial offices at all, the offices became a de facto political reality without a formal constitutional status. Some constitutions make a Prime Minister primus inter pares (first among equals) and that remains the practical reality for the Prime Minister of Belgium and the Prime Minister of Finland. Other states however, make their head of government a central and dominant figure within the cabinet system; Ireland's Taoiseach, for example, alone can decide when to seek a parliamentary dissolution, in contrast to other countries where this is a cabinet decision, with the Prime Minister just one member voting on the suggestion. In Israel, while the Government is nominally a collegiate body with a primus inter pares role for the Prime Minister, the Israeli Prime Minister the dominant figure in the executive branch in practice.[6] The Prime Minister of Sweden, under the 1974 Instrument of Government, is a constitutional office with all key executive powers at his disposal; either directly, or indirectly through the collegial Government; whose members are all appointed and dismissed at the Prime Minister's sole discretion.

The head of government is often provided with an official residence, often in the same fashion as heads of state often are. The name of the residence is often used as a metonym or alternate title for 'the government' when the office is politically the highest, e.g. in the UK "Downing Street announced today…"

Similarly, heads of government of (con)federal entities below the level of the sovereign state (often without an actual head of state, at least under international law) may also be given an official residence, sometimes used as an opportunity to display aspirations of statehood:

Usually, the residence of the heads of government is not as prestigious and grand as that of the head of state, even if the head of state only performs ceremonial duties. Even the formal representative of the head of state, such as a governor-general, may well be housed in a grander, palace-type residence. However, this is not the case when both positions are combined into one:

The White House (1600 Pennsylvania Avenue) in Washington, D.C. — President of the United States of America

The Blue House (1 Sejongno Jongno-gu) in Seoul — President of South Korea